A rotor blade of a wind turbine typically comprises a root section, which is arranged and prepared for being attached to a hub of the wind turbine, and an airfoil section, which is arranged and prepared for generating lift. Regarding the airfoil section, a pressure side, a suction side, a leading edge and a trailing edge can be attributed to it.
At every spanwise position of the rotor blade, a chord line, which is a straight line connecting the trailing edge and the leading edge, can be determined. The angle between the chord line and the direction of the airflow that is impinging on the rotor blade at the leading edge is referred to as the angle of attack. If the actual angle of attack equals a pre-determined design angle of attack, an optimum power can be extracted from the airflow that is impinging on the rotor blade. The rotor blade is designed such that for normal operating conditions of the rotor blade the airfoil shape and the structural characteristics are optimal.
However, various circumstances can lead to deviations of the actual angle of attack with regard to the design angle of attack. Reasons for these deviations are, for instance: turbulence variations in the airflow, thus the wind speed suddenly increases or decreases; gusts where the wind speed suddenly changes direction; changes in the angle of attack due to torsion of the blade; changes of the angle of attack due to wind shear and veer; operation of the wind turbine with a yaw error. When the angle of attack at which the rotor blade operates deviates from the optimal angle of attack, i.e. the design angle of attack, the lift which is generated by the rotor blade may deviate from its optimal value.
This deviation can, generally speaking, have two effects on the wind turbine:
First, there may occur a non-optimal power extraction due to either high loading which may cause a high blocking of the airflow by the wind turbine, or due to low loading and therefore resulting in a low torque.
Second, there may occur a higher loading of the components as the components have been designed to due to, for instance, a simultaneous extreme increase in wind speed and increase in the angle of attack.
Both effects are in principle unwanted effects as they reduce the energy output of the wind turbine, reduce the life time of the wind turbine, and/or cause an overly conservative design of the main components of the wind turbine.
Thus, it is desirable to optimize the lift that is generated by the rotor blade.
Several solutions are known for increasing or decreasing the lift of a rotor blade.
A first option is the provision of Gurney flaps which are typically add-ons that are attached at the trailing edge at the pressure side of the rotor blade. An example of such a modification, which may also be integrated in the design of the airfoil such that the attachment of a separate piece is not necessary, is given in the European patent application EP 2 004 989 A1.
A second option for influencing the lift of the rotor blade is the provision of flaps that are connected to or integrated into the trailing edge section of the rotor blade. Examples of such flaps are given in the Danish utility model DK 95 00009 U3. In this document, it is proposed to add a rigid or flexible flap at the pressure side at a trailing edge section behind the trailing edge or upstream of the trailing edge.
Yet another option to influence the lift of the rotor blade is the provision of slats or other add-ons that are mounted to the leading edge section of the rotor blade. The concept and some specific embodiments of this idea is for example disclosed in the European patent application EP 2 078 852 A2.
These aerodynamic add-ons may be externally actuated. In other words, there is a mechanism by which these aerodynamic devices can be actuated externally. Actuation may be implemented by mechanical mechanisms, piezoelectric mechanisms or hydraulic mechanisms. An example of a piezo-actuated lift modifying trailing edge section of a rotor blade is, for instance, disclosed in the European patent application EP 2 233 735 A2.
The conventional methods and devices for influencing the lift of the rotor blade, in particular for optimizing the lift of the rotor blade in order to maximize the energy output that the wind turbine can generate, have, however, several drawbacks.
On the one hand, there may be the need of external actuator mechanisms which may be costly to implement and expensive to service.
Additionally, it may be difficult to locally influence the lift of the rotor blade. In this context it is important to understand that it may occur that at a first section of the rotor blade the actual angle of attack approaches the design angle of attack well, while at a different, second section at the rotor blade the design angle and the actual angle of attacks differ. In other words, it may be beneficial to be able to locally influence the lift if needed. Another disadvantage of existing lift influencing devices is that typically they are able to either increase the lift or decrease the lift.
It is thus desirable to provide alternative means to influence the lift characteristics of a rotor blade, in particular of an airfoil section of the rotor blade. Preferably, these alternative means overcome at least some of the mentioned drawbacks of existing lift influencing devices.